@Kaarme Where'd martial arts even enter into this discussion? All I'm saying is that mundane firearms not backed by magic or special abilities of some kind would be useless against monsters, thus it's not really that big a deal to have them in the fantasy setting, even if it does make things anachronistic as heck. Though since primitive firearms were around in some form or other since the late middle ages, not all that anachronistic.
As for firearms in a "fantasy" setting. What defines fantasy, and what makes firearms inherently not fantasy? Just because Japan has been drowning in the standard medieval European, swords and sorcery, elves, dwarves, and dragons style of fantasy for forever and a half doesn't mean everything has to adhere to that template. While I would raise an eyebrow at seeing what looks like precision-machined rifle-barreled small arms in the possession of mere villagers in the apparent technological period of the setting, firearms themselves aren't that big of an issue. Heck, the discovery of gunpowder would have occurred sooner or later if they've got alchemists brewing up various kinds of potions, or just the concept of launching a projectile with the power of contained, directed force.
And sure, firearms and their effects aren't gonna be portrayed accurately. You know what else isn't portrayed accurately? Swordsmanship, archery, medieval architecture, the list goes on. At the end of the day, the writer/mangaka can't be a subject matter expert on everything, though it would certainly be nice if they at least have passing familiarity with the subjects they portray, and some realism inevitably has to give way to "rule of cool". So I'm content to go with good enough accuracy, or at least feasibility. Not saying you have to as well, since everyone has different tastes and different things that break their suspension of disbelief/tolerance of bullshit. Just saying not to write off certain aspects out-of-hand just because at first glance they seem to clash with the tone/setting.